Hybrid vehicles include an engine and an electric machine that operates as a motor/generator with an associated battery to provide an electric vehicle (EV) mode using only electric power to propel the vehicle, or a hybrid electric vehicle (HEV) mode that uses the engine and motor to propel the vehicle. Hybrid vehicles may start the engine using a dedicated starter motor and/or various types and sizes of electric machines that may function as a motor/generator or an integrated starter-generator (ISG). The engine may be started frequently under various operating conditions to transition between EV and HEV operating modes such that the selected starting strategy may significantly impact overall vehicle efficiency, performance, and drivability.
The electric machine of a hybrid vehicle may be coupled to the engine to start the engine under various operating conditions. However, this requires that the electric machine reserve sufficient torque capacity during operation to crank the engine and overcome engine friction and inertia such that the torque or battery power is not fully available to propel the vehicle when operating in EV mode. Depending on engine size and engine temperature, the torque reserved for cranking the engine during starting may be significant, e.g. from 80-150 Nm, which may represent up to 40% or more of the maximum torque available. In addition, coupling the engine to driveline components during starting may introduce torque disturbances that reduce vehicle efficiency and drivability. Similarly, vehicles that rely only on a dedicated starter or ISG may have disadvantages with respect to cost and efficiency.